It is well know in the art to use piezoelectric devices as ultrasonic transducers. For example ceramic-based piezoelectric lead zircomate titanate (PZT) is used in commercially available ultrasonic transducers. FIG. 1 illustrates a typical prior art PZT ultrasonic transducer, including a two-layer transducer element 10 with a poled PZT sheet 11 bonded with a metal sheet 12.
The transducer element 10 is fixed by silicone adhesive 14 on a ceramic support 13 within a case 18. A pair of pins 15 are electrically connected to the PZT layer 11 and are fixed in place though the ceramic support 13 by an adhesive 16. The transducer element 10 is deflected when an external voltage is supplied though the pins 15. This provides larger displacements around the centre of the element.
In order to intensify the transducer-to-air coupling efficiency, a lightweight cone 17 is attached to the centre of the transducer element 10. A number of sound emission holes 19 are provided in the case 18, in front of the cone 17. This device is good for many applications, but its parametric ability and maximum sound pressure level are limited due to its size.
When applied to a parametric audio system, such elements may be used in an array form using many elements for effective parametric conversion. FIG. 2 is an example of an ultrasonic transducer array employing this type of commercial PZT transducer elements for a parametric speaker. Four elements are displayed, all attached to one support frame 21 through pins 15. Electrical wiring is provided so that all the transducers 10 are connected in series, so they all respond equally and concurrently to an applied voltage. Building the ultrasonic transducer array in such a way is straightforward, but unfortunately there exist a few important technical problems.
Firstly, it is difficult to align all the transducer elements. Ideally the central axis (line c-c′ in FIG. 2) in each element would be perfectly perpendicular to the support frame 21 and all the elements would be at the same level. However since there are non-uniform adhesion points 16 sandwiched between the transducer's ceramic plate 13 and the support frame 21 this is highly difficult to achieve in practice. As a result, each transducer element has a slightly different phase which limits the overall performance.
Secondly, the case 18 or even the ceramic plate 13 of each transducer occupies substantial space, making it more difficult to provide a thin and compact array aperture for mobile device applications. Furthermore, the case 18 tends to increase the centre-to-centre distance between adjacent transducer elements, which is not conducive to suppressing the side-lobes of the transmitted ultrasonic wave.
Thirdly, placing commercial transducers onto a support frame one by one to form an array, and electrically connecting them one by one, limit manufacturability and decrease both uniformity and repeatability.